This invention relates generally to the control of mechanical vibration and more particularly has reference to the extensions and improvements of the vibration suppressor.
The extension is directed at solving the problem of automatically controlling the V.S. to provide the maximum impedance at the mounting point which is being driven by an oscillatory force whose frequency changes slowly with time. (I A, B, C).
This invention is an improvement on my previous invention disclosed in U.S. Pat. No. 3,088,062, for Electromechanical Vibratory Force Suppressor and Indicator, the disclosure of which is incorporated herein by reference. The specification for that patent describes a mechanical circuit (usually a spring s and a mass m in parallel) and a transducer with a particular external electrical termination Z.sub.e which has been selected by the operator (based on his estimation of the instantaneous frequency f which he enters into a previously determined "Z.sub.e vsf" chart). The electrical termination may be at a remote point. The transducer transforms the total electrical impedance composed of the blocked impedance Z.sub.T of the transducer in series with the external electrical impedance Z.sub.e into a virtual mechanical element whose impedance is Z.sub.v. This is shown in FIG. 1 (a copy of FIG. 1a of the patent) in an equivalent electrical circuit where F is the open circuit force of the noise source, Z.sub.i is the internal impedance of the noise source, Z.sub.b is the impedance of the base on which the noise source rests and G is the electromechanical coupling constant. As Z.sub.e changes, the resulting Z.sub.v will change the antiresonant frequency of the V.S., at which frequency the mechanical impedance at the base F/vb will remain very high. But, at first, magnitude of the impedance decreases as the shift in the antiresonant frequency increases. The theoretical basis of FIG. 1 is demonstrated in the Analysis portion of the Patent, Cols. 4, 5, 6, 7, and was verified with the experimental setup shown in FIG. 2 (Patent FIG. 1a) in which the vibration suppressor was mounted on a thin plate, supported on pillow blocks and which was driven by a ballistically suspended shaker unit. The pillow blocks were mounted on a heavy horizontal baseplate. The results are given in FIG. 3 (similar to Patent FIG. 18) in which the experimentally determined optimum external capacitances (o) at discrete frequencies are compared with the theoretical curve (--). In an experiment using the setup of FIG. 2, the relative velocity of suppression was determined as a function of frequency, the external terminating capacitance at each frequency being given by FIG. 3. The results are plotted in FIG. 4 (similar to Patent FIG. 17). If, on the other hand, a fixed external termination capacitor is used regardless of frequency, a greatly reduced velocity may be achieved at the frequency for which the given capacitor is the optimum termination, but, as the frequency moves either up or down, the velocity increases and may become larger than for the velocity without a suppressor (or dynamic absorber). The frequency-response curve is then said to have "EARS".
Results (--) are plotted in FIG. 5 for case without velocity control; it has a strong peak in velocity curve at a frequency as shown. For a vibration suppressor terminated optimally at this frequency, the response curve (- - -) shows a strong attenuation in velocity at that frequency, and the appearance of "EARS".
The kind of results obtained in FIG. 4 on a frequency-by-frequency basis can be obtained simultaneously over as wide a band of frequency as that over which the correct electrical termination can be achieved. FIG. 6 (corresponding to Patent FIG. 13) gives a positive feedback circuit of the "series" type for obtaining broad-band electrical termination.
A point is reached at which increasing the bandwidth is no longer productive, as may be seen in FIG. 12. A simpler, more stable circuit that can provide proper termination over the effective bandwidth would be more useful.
The vibration suppressor, with or without the line-following capability, is basically a dynamic absorber combined with a transducer in such a way that its electrical termination is transformed to a virtual mechanical element that then operates as part of the dynamic absorber. The efficacy of the vibration suppressor depends on making the energy loss of both the mechanical and the electromagnetic parts as small as possible. Thus, most of the quasiharmonic noise force has been cancelled, leaving a small net foundation velocity. This can be reduced much further by also using a negative-feedback velocity-control; this composite can be much more effective than the usual feedback alone. The negative feedback must be so applied that it does not interfere with the V.S. process.
Some of the transducer types considered for use with a mechanical antiresonant device in a V.S. exhibit nonlinear behavior. Push-pull arrangements have been used for (electrically driven) electrostatic speakers, and perhaps even for variable reluctance drivers. But what is proposed here is the inverse--to have a vibratory noise force applied to a push-pull transducer (e.g. variable reluctance type) at the mounting point. This will result in much greater linearity and in providing a more effective and cheaper dc current source for the large dc magnetic-field bias that is required.